1. Field of the Invention
This invention relates to measuring the performance of steam utilizing machines, and more particularly, to means for measuring steam quality or moisture at various points in a steam flow.
2. Description of the Prior Art
Determination of thermodynamic performance of steam utilizing apparatus requires measurement of thermodynamic state points at various points in the apparatus. Thermodynamic state points can be determined for dry steam from temperature and pressure measurements, but determination of state points for wet steam necessitates the ascertainment of the steam quality or moisture parameter. Steam turbines used for power generation require their thermodynamic and mechanical performance to be closely monitored. Thermodynamic performance monitoring provides an indication of the cost of turbine operation and gives early warning signals for many malfunctions. Steam quality in steam turbines is important to the mechanical performance since the moisture level at various locations in the turbine and at selected points along the blades is a very important parameter since moisture can cause blade erosion, corrosion-fatigue failures, and other mechanisms which can lead to mechanical failure of the turbine or its parts.
Calorimeters have, historically, been used to measure steam quality, but in recent years calorimetry has fallen into disrepute due to serious questions raised as to its accuracy. Aside from inaccuracy questions, calorimeters yield relatively slow quality determinations and, as such, cannot provide continuous quality monitoring. Such continuous monitoring is very important if quality measurement of steam in a thermodynamic machine is to be used for diagnostic and malfunction detection purposes.
A substantial part of the calorimeters' inherent inaccuracy results from the requirement of withdrawing a steam sample from the steam flow in question. Withdrawing such steam samples from the main steam flow exposes such withdrawn steam to a withdrawing probe permitting heat transfer therebetween and results in throttling of the steam sample between the main steam flow and the calorimeter or other instrument remotely situated from the main steam flow. An additional problem associated with such physically remote quality measuring devices is the necessity of being able to withdraw the steam sample from the steam vessel in a relatively straight line. Such straight line probe or quality measuring device restricts the turbine design and often necessitates a more complex assembly procedure than would otherwise be required. Other attempts to measure steam quality while avoiding withdrawing a steam sample include comparisons of light intensity penetrating the steam flow with those light intensities measured through dry steam flows. Such attempts often encountered "fogging" problems on transparent surfaces through which the measuring light beam normally passed. Such fogging resulted from steam condensation and caused inaccurate light intensity measurements.
Many of the problems associated with the prior art steam quality measuring apparatus stemmed directly from the straight line probe requirements and their inability to provide fast and continuous quality indications.